Feature/representor

@mszenher or @asmith-mdsol Please review and merge.

This PR adds a semantic decorator that knows how to extract semantic data from an object and uses it in an enumerator. Enumerators are defined to iterate over descriptors in anticipation of using them to more efficiently serialize a 'representor' object as different media types.

All specs passing:

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124 examples, 0 failures
Coverage report generated (99.89%) covered.